Elastomer polymers or their composites are widely used in medium voltage (1 kV to 38 kV) to high voltage (38 kV to 200 kV) cable splice and termination applications, where a combination of both decent insulating/dielectric properties and elastomer-type mechanical performance are required. However, the intrinsic low permittivity of rubber greatly impedes its ability in internal electrical-stress grading. One solution is to introduce high-k inorganic particles into polymer matrices to improve the polymer permittivity.
Many theoretical approaches are developed to predict the permittivity of composites. For instance, one of the most well-known equations, Lichtenecker's logarithmic mixing rule, is based on the assumption that every capacitive component is randomly dispersed in composites, obeying a statistical distribution of in-parallel connections and in-series connections. Based on a mean field theory, Maxwell-Wagner equation and Bruggeman model are derived, with the former one being only valid for the infinite dilution of dispersed phases (0-3 composites) and the latter one being effective to predict the effective dielectric constant of composites at a high filler concentration (0-0 composites). All these theories have successfully predicted a limited improvement in the dielectric constant at a small inorganic loading (fundamentally, the improvement comes from the intensified electric field in the polymer phase, with little contributed by the inorganic phase); however, as a guide for selecting the type and the concentration of ceramic fillers, the theories fall short of expectations.
One of the issues underlying the inaccurate estimate for the dielectric constant of composites is that they do not account for the effects of the interface between fillers and polymers, especially when filler size shrinks from micrometer to nanometer scale. Qualitatively, a multi-layered core model is proposed to describe the influence of interfaces on the physical and electrical properties of composites. Namely, the multicore structure of an interface consists of a bonded layer, a bound layer and a loose layer; additionally, an electric double layer overlaps the above three layers. The dielectric properties of polymer composites can be significantly altered when interfacial effects begin to dominate. The complex nature and the enriched structures of interfaces also complicate the quantitative prediction for the composite permittivity.
Beyond the permittivity/dielectric-constant, a high breakdown strength and a low loss are additional prerequisites of the dielectrics operating under high electrical stress. Namely, for insulation materials, high permittivity allows for better stress relief, because the accumulated electrical stress can cause local discharge or flashover, leading to the system failure. Ceramics usually possess a large dielectric constant and good thermal stability, but suffer from low breakdown strength, poor flexibility and challenging processing conditions. Polymers, on the other hand, offer a high electric breakdown field, but their applicability is largely impeded by their intrinsic low permittivities (ε′10). In addition, polymers possess many manufacturing advantages, including easy processing and large-scale fabrication with reduced cost, and they are also light and can exhibit good mechanical properties. Consequently, polymer based dielectric materials with a high dielectric constant and a low loss are very attractive in the insulation area and also for other capacitive applications.
From the composite point of view, traditional approaches to increase the dielectric constant of polymers are to introduce either high-permittivity ceramic fillers or conductive fillers. In the first approach, the magnitude of the improvement is very limited (around several times) at a reasonable filler loading, as can be predicted by the logarithmic mixing rule. Fundamentally, the great mismatch of the dielectric constant between fillers and polymers distorts and intensifies the internal electric field in the polymer matrix. As a result, the increase in the effective permittivity of these composites comes from the polymer polarization at an enhanced electric field, with very little from the ceramic phase. Highly inhomogeneous fields also reduce the effective breakdown strength of the composites. At high filler loadings, which are required to afford considerable enhancement in the permittivity, composites lose the desired mechanical properties and integrity.
In contrast to the first approach, the dielectric constant of the composites filled with conductive fillers could dramatically increase at a small filler concentration in the vicinity of, but below, the percolation threshold. Simply stated, conductive-particle/polymer composites exhibit an insulator-metal transition near the percolation threshold, which is characterized by an abrupt change in the conductivity and divergence in the permittivity above percolation, whereas they remain insulator below percolation. Structurally, the substantial increase in the dielectric constant arises from numerous microcapacitors (neighboring conductive particles separated by a thin dielectric layer in between). The large capacitance contributed by each of these microcapacitors is related with a significant increase in the local electric field when the conductive particles are close to each other near the percolation threshold. For instance, a dielectric constant of 400 with a weak frequency and temperature dependence was reported for Ni-PVDF composite when the Ni concentration approaches the percolation threshold. The two primary problems with the conductor-insulator composites are high dielectric losses and narrow processing window. Namely, (a) near the percolation threshold, there develop high internal electric field which result in increased motion of any available charge carriers, that is, a high dielectric loss. (b) the insulator-to-conductor changes are very abrupt, with steep slopes and abrupt transitions, within a narrow concentration range around the percolation transition, thus, precise control of the filler concentration and dispersion are required everywhere across the system for stable performance, a requirement which can be rather difficult especially in scaled-up manufacturing. Several strategies have thus been proposed to suppress the dielectric loss by avoiding the direct contact between conductive fillers. For example, an epoxy composite filled with the self-passivated aluminum particles enclosed by insulting Al2O3 layers could exhibit a high dielectric constant up to 100 and loss tangent as low as 0.02. In another study, silver nanoparticles were coated with thin organic shells, and these core-shell nanoparticles were used for the preparation of polymer composites that showed high permittivity of 400˜500 with low loss tangent less than 0.05.
Theoretically, since the permittivity (effective dielectric constant) is proportional to the cumulative dipole moment (the number of opposite charges times their displacement), the shape, size and spatial arrangement of fillers are expected to have a significant impact on the dielectric properties. Towards this end, much research relied on the varying the geometric features of individual fillers, such as 1D carbon nanotubes or 2D graphite platelets, but less effort was focused on the design of the composite structure to capitalize on well-defined filler clusters; that is, design of preferential filler distribution, filler/filler preferential location in multifiller composites, controlled-size clustering of a filler within domains separated by a second filler or matrix, etc.